feat: exclude merge commits from --range by default

README.md

@@ -160,6 +160,19 @@ commit-guard --range v1.0..v2.0
 commit-guard --range origin/main..HEAD --enable subject,imperative
 ```
 
+Merge commits are excluded by default. Use `--include-merges` to check them:
+
+```bash
+commit-guard --range origin/main..HEAD --include-merges
+```
+
+An empty range (no commits) exits non-zero by default — this catches
+misconfigured range specs in CI. Use `--allow-empty` to exit 0 instead:
+
+```bash
+commit-guard --range origin/main..HEAD --allow-empty
+```
+
 ### pre-commit
 
 Add to your `.pre-commit-config.yaml`:

src/git_commit_guard/__init__.py

@@ -224,10 +224,14 @@ def _get_message(rev):
         sys.exit(f"git error: {stderr}")
 
 
-def _get_range_revs(rev_range):
+def _get_range_revs(rev_range, *, include_merges=False):
+    cmd = ["git", "log", "--format=%H"]
+    if not include_merges:
+        cmd.append("--no-merges")
+    cmd.append(rev_range)
     try:
         output = subprocess.check_output(  # noqa: S603
-            ["git", "log", "--format=%H", rev_range],  # noqa: S607
+            cmd,
             text=True,
             stderr=subprocess.PIPE,
             timeout=GIT_TIMEOUT,
@@ -247,6 +251,8 @@ class Args:
     allowed_types: frozenset
     max_subject_length: int
     rev_range: str | None
+    allow_empty: bool
+    include_merges: bool
 
 
 def _resolve_enabled(args, config, parser):
@@ -350,13 +356,30 @@ def _parse_args():
         metavar="REF..REF",
         help="check all commits in the given revision range",
     )
+    parser.add_argument(
+        "--allow-empty",
+        action="store_true",
+        default=False,
+        help="exit 0 when --range yields no commits (default: exit 1)",
+    )
+    parser.add_argument(
+        "--include-merges",
+        action="store_true",
+        default=False,
+        help="include merge commits when checking a range (default: excluded)",
+    )
     args = parser.parse_args()
     config = _load_config()
     enabled = _resolve_enabled(args, config, parser)
     allowed_scopes, require_scope = _resolve_scopes(args, config)
     allowed_types = _resolve_types(args, config)
     max_subject_length = _resolve_max_subject_length(args, config)
 
+    if args.allow_empty and not args.rev_range:
+        parser.error("--allow-empty requires --range")
+    if args.include_merges and not args.rev_range:
+        parser.error("--include-merges requires --range")
+
     if args.rev_range:
         if args.rev is not None or args.message_file:
             parser.error("--range cannot be combined with rev or --message-file")
@@ -384,6 +407,8 @@ def _parse_args():
         allowed_types=allowed_types,
         max_subject_length=max_subject_length,
         rev_range=args.rev_range,
+        allow_empty=args.allow_empty,
+        include_merges=args.include_merges,
     )
 
 
@@ -430,10 +455,10 @@ def main():
         _ensure_nltk_data()
 
     if args.rev_range:
-        revs = _get_range_revs(args.rev_range)
+        revs = _get_range_revs(args.rev_range, include_merges=args.include_merges)
         if not revs:
             sys.stderr.write("no commits in range\n")
-            return 0
+            return 0 if args.allow_empty else 1
         failed = False
         for rev in revs:
             message = _strip_comments(_get_message(rev))

tests/test_git_commit_guard.py

@@ -894,14 +894,58 @@ def test_range_all_fail_returns_one(self):
         ):
             assert main() == 1
 
-    def test_range_empty_returns_zero(self, capsys):
+    def test_range_empty_returns_one(self, capsys):
         with (
             patch("sys.argv", ["cg", "--range", "origin/main..HEAD"]),
             patch("git_commit_guard._get_range_revs", return_value=[]),
+        ):
+            assert main() == 1
+        assert "no commits in range" in capsys.readouterr().err
+
+    def test_range_empty_with_allow_empty_returns_zero(self, capsys):
+        with (
+            patch("sys.argv", ["cg", "--range", "origin/main..HEAD", "--allow-empty"]),
+            patch("git_commit_guard._get_range_revs", return_value=[]),
         ):
             assert main() == 0
         assert "no commits in range" in capsys.readouterr().err
 
+    def test_allow_empty_without_range_exits(self):
+        with (
+            patch("sys.argv", ["cg", "--allow-empty"]),
+            pytest.raises(SystemExit),
+        ):
+            main()
+
+    def test_include_merges_without_range_exits(self):
+        with (
+            patch("sys.argv", ["cg", "--include-merges"]),
+            pytest.raises(SystemExit),
+        ):
+            main()
+
+    def test_range_include_merges_flag(self):
+        with (
+            patch(
+                "sys.argv",
+                [
+                    "cg",
+                    "--range",
+                    "origin/main..HEAD",
+                    "--include-merges",
+                    "--disable",
+                    "signature",
+                ],
+            ),
+            patch(
+                "git_commit_guard._get_range_revs",
+                return_value=["abc1234"],
+            ) as mock,
+            patch("git_commit_guard._get_message", return_value=_VALID_MSG),
+        ):
+            main()
+        mock.assert_called_once_with("origin/main..HEAD", include_merges=True)
+
     def test_range_conflicts_with_rev(self):
         with (
             patch("sys.argv", ["cg", "abc123", "--range", "origin/main..HEAD"]),
@@ -930,6 +974,16 @@ def test_returns_shas(self):
         ):
             assert _get_range_revs("origin/main..HEAD") == ["abc1234", "def5678"]
 
+    def test_excludes_merges_by_default(self):
+        with patch("git_commit_guard.subprocess.check_output", return_value="") as mock:
+            _get_range_revs("origin/main..HEAD")
+        assert "--no-merges" in mock.call_args[0][0]
+
+    def test_includes_merges_when_requested(self):
+        with patch("git_commit_guard.subprocess.check_output", return_value="") as mock:
+            _get_range_revs("origin/main..HEAD", include_merges=True)
+        assert "--no-merges" not in mock.call_args[0][0]
+
     def test_empty_range_returns_empty_list(self):
         with patch("git_commit_guard.subprocess.check_output", return_value=""):
             assert _get_range_revs("origin/main..HEAD") == []